Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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"ELIMINATING ~LOW WRINKLES IN COMPRESSION MOLDED PANELS"
FIELD OF THE INVENTION
This invention relates to a process for the m~nllf~cnlre of
products from reinforced thermoplastic materials, more particularly to a
method to eliminate flow rippling in molded reinforced thermoplastic parts
o having large surface areas.
BACKGROUND OF THE INVE~ON
Plastics continue to replace metals as materials of
construclion. The cost savings of plastics over metals combined with the
ease of manufacturing and weight savings provide combined impetus for
plastic's invasion into the metal field.
Plastics can be fabricated into a variety of geometric designs
possessing a wide range of strengths. Indeed, plastics can now be
fabricated as replacements for external, semistructural metal members such
as automobile hoods, deck lids, roofs and fender extensions. Reinforced
2~ thermoplastic molding compounds have shown increased acceptance in
recent years due to their lighter weight, lack of outgasing during painting
and ease of recyclability. This is in contrast to thermosetting compounds
uhich do not possess the aforementioned attributes.
Of the conventional methods of making plastic parts,
2~ including semistructural plastic parts, automatic and semiautomatic
moldino techniques have proven most popular because of their high
production rates, lower manpower costs and improved product uniformity.
One of the more popular semiautomatic molding processes is the method
lino~ n as compression molding. The mold charge is molded upon closing
30 of the mold members as the material flows into the open cavity. Smooth,
aligned surfaces, about the perimeter of the male and female mold
members, called shear edges, slide together and effect a "pinch off" around
the perimeter of the mold to stop the major flow of molding compound
from the mold cavity. The edge opening, or shear edge gap, located at the
3~ peripheral edges of the mold, is typically on the order of 0.0508 to 0.127 mm(0.002 to 0.00~ inch) in width. The gap is purposely kept very small, i.e. just
large enough to permit closing of the mold without binding and small
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enough to prevent any substantial escape of the molding charge from the
molding cavity. Unfortunately, in using the small shear edge gap, the
surface of such parts, as molded, have an unacceptable appearance and
must be hand finished. Wrinkling of the surface is apparent.
During the mold filling step, pressure forces the molding
compound to flow and fill out the cavity of the mold. When the molding
compound reaches the shear edge, it is very close to the freezing or
solidification point of the resin. As the flow impacts the shear edge "wall",
the residual energy associated with the flow causes the molding compound
to rebound and attempt to flow back into the oncoming flow front, in turn
bucl;]ing reinforcing material in the molding compound. The buckled
reinforcine material becomes frozen in the solidifying matrix resin. As the
composition continues to solidify, the buckled reinforcing material begins to
show. through to the surface of the molded part. This nflow rippling"
typica]lv manifests itself as a series of waveswith amplitudes as high as 25
microns and a frequency of 1500 to 3000 microns. It generally occurs at or
near the shear edges in a compression molded part and is visible to the
na};ed eye both before and after painting. Consequently, products with such
surface deformations must be hand-finished prior to painting, which is
2() exF~ensive and time-concuming Even with hand-finishing, the scrap rate for
such mol(led parts mav be as high as 50 percent.
Flow ripples have been documented in several different mold
configurations, e.g.. round, square and contoured parts. Attempts to solve
this problem have been made by altering the resin used as well as changing
2~ the reinforcing materials. Statistical testing of the major processing
variables showed no significant effect on the magnitude or occurrence of
the flow ripples present in the final molded part.
SUMMARY OF THE INVENTION
Pursuant to the present invention, reinforced thermoplastic
3() par~s are compression molded which are free from flow ripple patterns .
Significant post finishing is eliminated with a corresponding increase in
productivity and a decrease in cost for a final product.
In carrying out the present invention, a molding charge
comprised of a pluralitv of layers of randomly dispersed high modulus
3s reinforcing fibers held together by a solidified thermoplastic resin matrix is
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heated above the melting point of the resin and placed within a set of
cooperating dies which form a mold cavity. The peripheral edges of the
surfaces are spaced apart to provide a substantial shear gap around the
periphery of the mold. When brought together during the molding process,
the pressure generated in the molding dies forces the molding charge to
flow and fill the cavity of the mold. A flow front is formed and a quantity of
the molding charge is forced into the shear edge at the periphery of the
dies. The shear edge is large enough to receive a quantity of the molding
charge which is capable of dissipating residual energy from the flow front as
the molding material solidifies. If the shear edge is too small, the molding
material rebounds from the shear edge into the oncoming flow front of the
material. This rebound movement buckles the reinforcing component
w ithin the flowing material. The buc}iled reinforcement reads through the
finished molde(3 part as flow ripples.
If flo~ ripples are observed, the shear edge is adjusted to
increase the gap between the cooperating die edges. The shear edge gap is
increased to allow less hindered flow of molding material from the cavity of
the mo]d into the shear gap as the resin material solidifies. The size of the
gap required is dependent upon the characteristics of the components of
2() the molding charge, the mold temperature, the mold closing speed and the
required molding composition pressure.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of the upper and lower dies of
a mold showing a molding charge placed upon the molding surface of the
lo~ er die;
FIG. 2 is a schematic representation of the mold of FIG. 1 in a
partiallv closed position; and
FIG. 3 is a schematic representation of the mold of FIG. 1 in the
closed position showing the molding charge flowing into the shear gap at
.(~ the edge of the mold.
DETAILED DESCRIPTION QF THE INVENTTON
Referring to the drawingst FIG. 1 shows mold 10 having an upper
die 12 and a lower die 14 with opposing molding surfaces 22 and 24
cooperating to define molding cavity 16. Molding surfaces 22 and 24
3~ define mold cavity 16 and determine the shape of the part to be fabricated.
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Upper die 12 is typically connected to a mobile platen, not shown,
operating under the action of a ram or like device to control relative
movement between upper die 12 and lower die 14. The manner of
controlling the die movement is well within the skill of the ordinary
5 practitioner.
In the open position, dies 12 and 14 are spaced apart to allow
mold charge 26 to be placed on molding surface 24 of lower die 14. The
term "charge" refers to a material cont~ining reinforcing material, other
adjuvants such as fillers, antioxidants and pigments, and one or more
thermoplastic resins that solidifies to provide a rigid, solid article. The resin
that mav be used can be anv material that provides the necessary bonding
and strength for the article to be formed such as thermoplastic polyesters,,
polyethylene, polypropylene, copolyesters, polyamides, including Nylon 6,
Nylon 6/6, Nylon 11, Nylon 12, J2, polyetherertherketone (PEEK),
polyether}~etoneketone (PEKK), polymethyphenylene, polyarylates and
polvvinylidene fluoride. These resins can be combined with reinforcing
fibers or fillers at concentrations from about S~G to 50~G by weight by a
ariety of methods to produce a reliable high modulus molded panel.
Preferred resin materials are thermoplastic polyester resins.
'() Reinforcing material may be any high modulus fiber such as
polvamides, glass, carbon, polyesters and high temperature nylons. Fibers
having a modulus of at least 100,000 M Pascals are preferred.
The present invention finds particular utility for compression
molding charges in the form of one or more sheets of sheet molding
compound materials of the type disclosed in Geary and Wee};s, US Patem
No 5,134,016 and European Patent No. 0 341 977 granted February 16,
1994, the disclosures of which are incorporated herein by reference. Such
material includes a thermoplastic resin mixed with reinforcing fibers and
various fillers to form a porous semi solid sheet or plv that can be cut into
3u desired shapes and placed in a mold. Typical reinforcing fibers include
polyimide fibers, polyester fibers, polyarnide fibers, natural fibers and metal
fibers. Preferably, the fibers are glass fiber strands.
In FIG. 2, mold charge 26 is formed from sheets of fiber reinforced
thermoplastic material. The thickness, weight and placement of the charge
3~ on the molding surface will depend on the desired configuration of the final
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part. In general, the charge should be centrally located with little or no
space being left between individual sheets if multiple sheets are used. Mold
charge 26 is heated to a temperature above the melting point of the matrix
resin by using a forced hot air convection oven or other suitable preheating
S technology. Heated mold charge 26 is then placed on molflin,~ surface 24 of
mold 16.
As shown in FIG. 2, dies 14 and 16 are moved toward each other to
a fully closed position. During the mold closing step, upper die 12 contacts
mold charge 26, causing it to spread and fill mold cavity 16. Mold charge 26
fills cavitv 16 and flows into shear gap 34 as shown in FIG. 3. The point in
time when mold charge 26 reaches the edge of shear gap 34 is very close to
the freezing or solidification point of the matrix resin of mold charge 26.
By controlling the width of shear gap 34 the residual energy associated with
the flowing resin is dissipated in shear gap 34 and the flowing resin does not
s rebound into the flow front of the mold charge. The width of shear edgegap 34 is dependent upon the composition of the mold charge used,
including the thermoplastic resin and the concentration of the reinforcing
fiber and filler. The size of the shear gap cannot be too small since this will
not allow for dissipation of the residual energy of the molding material as it
~(~ flows through the cavitv and reaches the shear gap. Likewise, an oversized
shear gap will not allow the proper buildup of pressure within the mold
needed to consolidate the part resulting in an excessive amount of charge
material 26 being forced out of molding cavity 16. The width of the shear
gap for a polvethylene terephthalate containing molding composition is
7~ generallv between 0.381 to 0.762 mm (0.015 to 0.030 inch).
After the resin solidifies sufficiently to permit handling,
molds 14 and 16 are separated to permit ejection or removal of the molded
part. Tvpicallv, the mold dwell time for a part 0.254 mm (0.100 inch) thick
is from 20 to 60 seconds.
The invention will be further illustrated by the following
indicated. Units reported throughout the specification and claims in Sl
units have been converted from the Fnglich system to the SI System.
EXAMPLE
A self-supporting porous batt was prepared from
3~ polyethylene terephthalate (PET) fiber having an as spun denier of 1.5. The
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fiber uas converted to a staple length of 0.635 cm and intim~tely mixed with
1.27 mm (0.5 inch) long glass fibers having a diameter of about 0.013 mm.
The above ingredients were slurried in water and collected as a mat on a
moving belt from a paper machine head box, then conveyed to a dryer. The
mat consisted of 25~o (nomin~l) glass fiber (Owens Corning Fiberglass '~"),
74.75~7c PET fiber and 0.25 % 'lrganox" 330 (Ciba Geigy antioxidant). A
belt speed of 7.62 m/m and a dryer temperature of 280~ C was used to
premelt the PET fibers into droplets, distributed along the glass filbers. A
sheet having a basis weight of 0.444 kg/sqcm was obtained.
o A cutting die was used to convert the sheet material into
17.78cm (7.0 inches) diameter circular pieces to cover approximately 48
of the net surface area of a compression mold. Numerous pieces were
stacked to achieve a preform weight of 235 g (nominal).
The preform was placed as a single stack in a convection
oven. Air heated to 320~C was forced through the stack at a rate of 106.6â
m/min for 60 seconds to melt the PET resin. The preform was then
transferred to a 25.4 cm (10 inches) diameter compression mold for 1
minute at a nominal mold temperature of 170~ C The mold pressure was
controlled at 13.7891 MPa (2000pSi). The final mold closure rate was
2.032 m/min (80 inches/min). The mold was then opened and the part
removed and allowed to cool.
The procedure described above was repeated except the
sheet material was cut into 15.24X15.24 cm (6 inches X 6inches) square
preform pieces covering approximately 48~c of the net surface area of the
compression mold.
A stylus type profilometer (Sutronic Model No. 3 with a
stylus head and probe having a diameter of 0.1016 mm (0.004 inch) was
used to measure the amplitude and frequency of any ripples formed in the
molded parts.
3(J The measurements obtained are shown in Table 1.
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TABLE 1
SHEAR
EDGE RING GAP AVERAGE AVERAGE
5CHARGEDL~METERDISTANCEFREQUENCYAMPLITUDE
SHAPE cm (inch~mm (inch~mm (inch~mm (inch~
Round 25.410.0 0.000 3.327 10.131 0.0114 0.00015
Square 25.410.0 0.000 3.150 0.124 0.0086 0.00034
Round 25.43 10.010 0.127 0.005 2.791 0.110 0.0083 0.00033
Square 25.43 10.010 0.127 0.005 3.124 0.123 0.0081 0.00032
Round 25.45 10.020 0.251 0.010 3.505 0.138 0.0053 0.00021
Squarc 25.4j 10.020 0.251 0.010 2.515 0.099 0.0081 0.00032
Round 25.18 10.030 0.381 0.015 1.321 0.052 0.0036 0.0011
Squarc 25.48 10.030 0.508 0.015 3.378 0.133 0.0056 0.00022
Round 25.:10.010 0.508 0.020 0 Flat
Squarc 2i.510.040 0.508 0.020 0 Flat
Round 25.53 10.0j()0.635 0.025 0 Flat
Squarc 2 j.53 1 ().05() 0.635 0.025 0 Flat
Roul1d 25.5j 10.()G()0.762 0.030 0 Flat
Squarc 25.55 10.060 0.762 0.030 0 Flat
Parls made according to the present invention sho~ no
~rin};les and have a smooth surface.
